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Variable rate fertilization technology is an important means of scientific fertilization, which can make fertilization more accurate and targeted, and effectively reduce farmland pollution. During high-speed rice transplanting and synchronous fertilization operations, the adjustment of fertilization amount is mainly controlled by pre calibration, which is time-consuming and unstable in accuracy. To quickly and accurately adjust the fertilization amount and achieve variable rate fertilization operations, an automatic control variable rate fertilizer application device was studied and designed for solid particle fertilizer. The overall structure and working principle of the variable rate fertilizer application device was expounded, and the key components were designed and tested. The on-line monitoring and intelligent control system of fertilizer amount were constructed with the single-chip microcomputer STM32 as the control core. By using the optimization method of experimental design, the performance and main influencing factors of the fertilizer flow online detection system were studied, and the best combination of factors was determined. Through the experiment, the change rule models between the detected flow of three kinds of main solid particle fertilizers and the voltage value of piezoelectric sheet, the change rule models between the actual flow of three kinds of main solid particle fertilizers and the rotation speed of the fertilizer discharge shaft, the change rule models between the rotation speed of the fertilizer discharge shaft and the working length of the electric push rod and the forward speed of the rice transplanter were respectively constructed, and the models were tested and analyzed. The results showed that when the forward speed of the rice transplanter was 1m/s and the rotation speeds of the fertilizer discharge shaft were 20r/min, 25r/min and 30r/min, the average detection accuracy of the fertilizer discharge of the three solid granular fertilizers were 94.45%, 93.85% and 93.15%, respectively. When the forward speed of the transplanting machine was 0.6~1.4m/s, the compound fertilizer application rates were 200kg/hm2, 250kg/hm2 and 300kg/hm2 and the urea application rates were 165kg/hm2,195kg/hm2 and 225kg/hm2,the average coefficient of variation of the fertilization rates of the three fertilizers were 3.02%, 3.15% and 2.82%, respectively. Field experiment showed that when the transplanting machine advanced at speed of 1~1.4m/s and the fertilization rates were 180kg/hm2 and 225kg/hm2, respectively, the average accuracy rates of compound fertilizer fertilization in Norway were 94.89% and 97.82%, respectively. Theoretical and experimental analysis showed that the variable rate fertilization device had stable regulation performance and can meet the operational requirements of rice side deep variable rate fertilization.